The present invention relates to a lock-up control of a torque converter, used in the automatic transmission system of a vehicle.
Torque converters carry out the functions of absorbing torque fluctuations or increasing torque in order to transmit the motive force between the input/output elements through a fluid.
Since their transmission efficiency is lower than the conventional friction clutch, during running conditions where absorbing torque fluctuations or increasing torque are not necessary, the input/output elements of the torque converter are directly connected and put in a lock-up state.
In vehicles provided with an automatic transmission and a torque converter equipped with such a lock-up clutch, the torque converter is generally put in the lock-up state during coast running in order to increase the fuel cut period.
However when shifting from coast running to power running by depressing the accelerator pedal while the torque converter is maintained in the lock-up state, the torque converter may experience a torque shock due to sudden increase of input torque.
In order to suppress this torque shock, Tokkai Hei, 8-233098 published in 1996 by the Japanese Patent Office discloses the unlocking of the torque converter when shifting from coast running to power running is performed.
However in this conventional technique, if shifting between coast running and power running is frequently repeated during driving, the torque converter will be repeatedly locked up and unlocked. Thus the clutch facing of the lock-up clutch will soon wear out and this will adversely affect to the durability of the torque converter.
It is therefore an object of this invention to avoid frequent lock-up and unlock of the torque converter, while also avoiding torque shock when shifting from coast running to power running.
In order to achieve the above object, this invention provides a lock-up control device for a torque converter in a vehicle. The torque converter comprises an input element connected to an engine which rotates according to a depression of an accelerator and an output element connected to an automatic transmission. The lock-up control device comprises a sensor for detecting a depression degree of the accelerator, a sensor for detecting a vehicle speed, and a microprocessor programmed to determine whether or not the vehicle is coasting based on the depression degree of the accelerator, determine whether or not the torque converter is in a lock-up state wherein the input and output elements are directly combined, calculate an accelerator depression speed from the depression degree of the accelerator, shift the torque converter to an unlock state in which the input and output elements are indirectly combined when the vehicle is coasting, the torque converter is in the lock-up state, and the accelerator depression speed is larger than a predetermined speed, determine whether or not the vehicle speed is within a predetermined re-lock-up prevention speed range, and prevent the torque converter from shifting to the lock-up state when the torque converter has been shifted to the unlock state and the vehicle speed is within the predetermined re-lock-up prevention speed range.
When the engine comprises a throttle which operates according to the depression of the accelerator, it is preferable that the accelerator depression degree sensor comprises a sensor for detecting an opening of the throttle.
It is also preferable that the microprocessor is further programmed not to shift the torque converter to the lock-up state when the vehicle speed is less then the predetermined re-lock-up prevention speed range irrespective of the accelerator depression speed.
It is also preferable that the microprocessor is further programmed to learn an average speed of the accelerator depression from coasting of the vehicle, and modify the re-lock-up prevention speed range to a larger range when the average speed is larger than a standard value.
In this case, it is further preferable that the microprocessor is further programmed to set the larger range by modifying an upper limit of the re-lock-up prevention speed range to a larger value.
It is also preferable that the microprocessor is further programmed to set the larger range by modifying a lower limit of the re-lock-up prevention speed range to a smaller value.
It is also preferable that the microprocessor is further programmed to calculate a time elapsed from when the torque converter was shifted from the lock-up state to the unlock state, and allow the torque converter to shift to the lock-up state when the elapsed time has reached a set time even when the vehicle speed is within the predetermined re-lock-up prevention speed range.
In this case, it is further preferable that the microprocessor is further programmed to learn an average speed of the accelerator depression from coasting of the vehicle, and modify the set time to a longer time when the average speed is larger than a standard value.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.